1. Field of the Invention
The subject invention relates generally to temperature controllers and more particularly to a precision electronic temperature achievement controller.
2. Description of Prior Art
The increasingly stringent emission control standards being imposed on automobile manufacturers and the like by both federal and state governments have necessitated increasingly accurate testing apparatus for determining compliance with such standards. One recently promulgated emission test is the so-called "Diurnal Breathing Loss Test" (Federal Register requirement 86.133-78) which pertains to the measurement of fuel vapor emissions from automobile fuel tanks. The test requires the simulation of the heating of a fuel system experienced by a vehicle parked in a heating environment, (i.e., the warming of the vehicle fuel system while parked outdoors in the sun) following cold soaking through the night. The test is carried out by placing the fuel system to be tested in an enclosure which is commonly called a "SHED" (Sealed Housing for Evaporative Determination). The system under test is then heated by way of a heating blanket or the like at a precisely controlled rate from a predetermined base temperature to a predetermined final temperature. When the final temperature is achieved, the quantity of fuel vapors emitted by the fuel system, which is entrapped in the SHED, is measured to determine compliance.
Temperature achievement controllers are used to measure the actual temperature of the fuel system as it is heated and to control the heat output of the heat blanket is response thereto so that the desired rate of temperature increase is achieved. The prior art controllers have generally been adequate for use in conducting most of the early emission control tests. However, such controllers are believed to be inadequate for conducting many of the more recent emission tests such as the above-noted "Diurnal Breathing Loss Test" which requires, inter alia, that the rate of temperature increase be precisely maintained. One of the primary reasons for the limitations in the prior art controllers is the use in such controllers of proportional control circuits which control the amplitude of the voltage applied to the heating blanket. Proportional control circuits, especially in relatively high power applications, are generally incapable of accurately and rapidly varying the power applied to the heating blanket. Accordingly, the achieved temperature of the fuel system under test will tend to alternately undershoot and overshoot the desired temperature in excess of the maximum specified temperature deviation.
The present invention overcomes most of the limitations of the prior art temperature achievement controllers. The subject controller avoids the exclusive use of proportional control circuitry and is capable of maintaining the temperature of the fuel system under test within a fraction of a degree of the desired temperature. The subject controller is also provided with means for automatically disconnecting the blanket heating power when the temperature of the system under test exceeds a predetermined maximum. Alarms, both audible and visual, are incorporated which provide the operator with a warning of conditions that require his attention. Other advantages of the subject invention over the prior art will become apparent.